This invention relates to a process for making fast sodium-ion transport compositions.
As disclosed in U.S. Pat. No. 4,049,891, Hong et al., compounds of the formula: EQU Na.sub.n X.sub.2 (ZO.sub.4).sub.3
wherein n is a number greater than 1, X is an octahedrally coordinated cation or mixture thereof and Z is a tetrahedrally coordinated cation or mixture thereof have excellent properties which permit their use as fast sodium ion transport compositions. Prior to that invention, .beta. and .beta."-alumina were the solids with fastest sodium-ion transport; they stimulated interest in their use as solid electroytes for cells and thermoelectric generators as disclosed in U.S. Pat. No. 3,475,223. The best .beta."-alumina compositions have resistivities for sodium-ion transport at 300.degree. C. of about 4 ohm-cm and an activation energy for the mobility of about 0.16 eV. Unfortunately, the volatility of soda together with the refractory nature of aluminum oxide has rendered production of ceramic membranes therefrom expensive. Furthermore, .beta.- and .beta."-alumina are layer compounds in which the sodium ions are constrained to move in only two dimensions so that the confinement of sodium ions to widely separated layers sharply reduces the fraction of the membrane volume that transports sodium ions. Furthermore, the structure of .beta.- and .beta."-alumina promotes anisotropic thermal expansion so so that the life of thermally cycled membranes made therefrom is reduced significantly.
The Hong et al compositions are characterized by a three-dimensional rigid structure of open networks formed from corner-shared oxygen tetrahedra or tetrahedra and octahedra through which the Na.sup.+ ion can move in three dimensions. In addition, the minimum cross-sectional diameter of the interstitial space is about twice the sum of the atomic diameter of the anion and the sodium ion. Thus, the minimum cross-section diameter is about 4.8 A for sodium ions and usually about 4.8 to 5.4 A for sodium. The compositions are also characterized by having the lattice sites of the network interstitial space only partially occupied with sodium ions to permit ion transport within the crystal structure. If these sites were fully occupied by the sodium ion, there would be little or no sodium transfer within the crystal structure. In addition, if the sites available to the sodium ions are crystallographically inequivalent, the difference between the respective site-occupancy energies should be small and/or the number of mobile ions should be large enough to insure some occupancy of all types of sites available to them in order to minimize the activation energy of the mobility.
The compositions are useful as solid electrolytes in cells utilizing liquid sodium metal as a negative electrode wherein the composition comprises a membrane between the liquid metal and a positive electrode, such as liquid polysulfide impregnated with an electron conductor like carbon felt as in NaS battery. The compositions also can be utilized as a solid electrolyte in a thermoelectric generator employing a differential pressure of sodium ions maintained across the membrane formed of the composition. In addition, a membrane of these compositions can be utilized to extract selectively sodium ions from sodium salts such as NaCl.
As disclosed by Hong et al, the reactants utilized comprise the oxides and/or salts of the cations that are reducible to the oxides under the reaction conditions of temperature and pressure employed and the compositions are formed in a conventional manner including a procedure involving two heating steps. The starting materials in particulate solid form are heated to a sintering temperature for a period of time to assure conversion to a product wherein the anions and the cations are bonded to oxygen atoms to form the tetrahedral or tetrahedral-octahedral crystalline structure. For example, the solid mixture is heated stepwise to decompose the least thermally stable reactant such as NH.sub.4 H.sub.2 PO.sub.4 at about 170.degree. C. and then heating at an elevated temperature of about 900.degree. C. to decompose the Na.sub.2 CO.sub.3 and then at a highly elevated temperature of about 1200.degree. C. to transform the reactants, which form the desired crystalline structure. The period of heating depends upon the amount of reactants with a representative time period being between about 4 hours and 24 hours. When forming a ceramic structure from the compositions of this invention, a flux material can be added to the reactants and reacted therewith by any means well known in the art.
This method for forming these compositions described above is time consuming and expensive. In addition, for the preferred compositions of the general formula: EQU Na.sub.i+x Zr.sub.2 Si.sub.x P.sub.3-x O.sub.12
wherein x is greater than 0 and less than 3, the procedure utilized and required at least three starting materials, e.g., Na.sub.2 CO.sub.3, ZrO.sub.2, SiO.sub.2 and NH.sub.4 H.sub.2 PO.sub.4 and two heating steps and an overall time of about 12 hours in order to form the desired product. Prior attempts to form this product from the two inexpensive materials, zircon flour (ZrSiO.sub.4, ground fine) and hydrated trisodium phosphate (Na.sub.3 PO.sub.4) failed. Upon heating of these two materials, the trisodium phosphate appears to decompose rather than to react with the zircon flour. In any event, the desired product was not obtained.
It would be highly desirable to provide a process for making these fast sodium-ion exchange compositions from inexpensive starting materials and by a one-step process.